Cryptosporidium parvum infection in AIDS patients is characterized by profuse watery diarrhea, dehydration, electrolyte imbalances and severe wasting. the illness is often chronic in nature, and since no effective treatment is available, the infection usually continues until death. It has been estimated that 4% of AIDS patients develop cryptosporidiosis. C. parvum is an intracellular opportunistic protozoan that has a complicated life cycle that includes multiple asexual followed by sexual endogenous stages of reproduction. The first important C. parvum-enterocyte interaction occurs when sporozoites are released from oocysts and invade host enterocytes. Also, other life stages are released into the intestinal lumen where they again invade uninfected epithelial cells. We propose to study the mechanisms involved in C. parvum sporozoite adhesion and invasion of enterocytes. First, in vitro assays will be developed using radiolabeled sporozoites or monoclonal antibodies to surface antigens of C. parvum, so that the events of adhesion and invasion can be quantified. The metabolism of sporozoites and enterocytes will be altered, and the environmental conditions will be manipulated in vitro, to better define requirements for these biological processes. Scanning and transmission electron microscopy will be used to three-dimensionally reconstruct the events of adhesion and invasion. The nature of the age-related susceptibility of C. parvum infection will be explored using suckling and adult BALB/c mice as models of infection. We will attempt to identify surface receptors on C. parvum and enterocytes that are responsible of adhesion and invasion and develop inhibitors to these receptors. It is hypothesized that N-acetyl-B- glucosaminadase (NAGase) on the surface of enterocytes may be a critical cryptosporidial sporozoite receptor. Experiments are designed to test this hypothesis. The ability of interferons to inhibit cryptosporidial adhesion and invasion will be evaluated in an in vitro model of C. parvum infection. Our last goal will be to test the in vivo efficacy of identified inhibitors of C. parvum infection using our characterized suckling severe combined immunodeficiency (SCID) mouse model. The objectives of this proposal are to allow me to demonstrate the merit of my research ideas so that I will be competitive for more traditional types of research grants in the future. This work may provide insights into possible new treatment strategies for AIDS patients with cryptosporidiosis, including chitin, monoclonal antibodies to NAGase and interferons.